The full error is:
ImportError: Could not import settings 'app.settings' (Is it on sys.path?): No module named settings
The whole time,
python manage.py runserver
had been working fine, and DJANGO_SETTINGS_MODULE was never defined previously - that is not the issue here, since I'm running manage.py which should set this itself.
The problem occured after I added some imports - runserver immediately stopped working.   I also can't do validate - nothing works.  Setting DJANGO_SETTINGS_MODULE doesn't help, either, although that's what the error is suggesting.